Soft contact lenses offer a viable alternative to spectacles for the correction of visual defects such as myopia, hypermetropia and astigmatism. Since the pioneering work of Wichterle and Lim, hydrogel soft contact lenses have become a viable alternative to spectacles for the refractive correction of myopia, hyperopia and astigmatism. Hydrogel polymers, which make up the majority of lenses sold worldwide, are based on copolymers of 2 hydroxy ethyl methacrylate (HEMA). Although these lenses provide some comfort, they do not provide sufficient oxygen permeability to prevent problems associated with corneal hypoxia. Attempts to address this problem included copolymerising HEMA with hydrophilic monomers such as methacrylic acid acrylamide, poly (vinyl alcohols) and n vinyl lactams such as N-vinyl pyrrolidone. Although these polymers increased the level of oxygen permeability, the incorporation of these comonomers also leads to problems such as protein and lipid deposition, corneal desiccation, staining and lens dehydration.
More recently, a new generation of polymers has been developed to further increase the level of oxygen. These materials are based on the copolymerisation of silicone methacrylate with hydrophilic comonomers. Although successful in further increasing the oxygen permeability, these new materials still suffer from limitations such as lipid binding and dryness, all of which decrease lens on eye comfort.
The use of silicone-containing polymers has led to contact lenses exhibiting much higher oxygen permeabilities. However, the incorporation of silicone can lead to other adverse performance characteristics, such as surface wettability and opacity problems.
Silicone-containing materials suitable for contact lens technology are described in U.S. Pat. No. 6,312,706, which discloses a hydrogel material that is the polymerisation product of a comonomer mixture comprising (a) a polysiloxane-containing urethane prepolymer end-capped with polymerizable ethylenically unsaturated organic radicals, (b) tris-(trimethylsiloxy)silyl propyl methacrylate and (c) a hydrophilic comonomer.
U.S. Pat. No. 4,136,250 teaches polymers formed from mono-olefinic monomers cross-linked with a major amount of a di- or tri-olefinic polysiloxane based macromer having a molecular weight between about 400 and about 800.
U.S. Pat. No. 4,962,178 discloses siloxane-urethane polymers suitable for use as oxygen permeable membranes or ophthalmic devices, having based on total urethane groups 50-80% of —C—NH—COO—C— groups and 50-20% of —C—NH—COO—Si— groups, which consists essentially of the polymerisation product of (a) 80-95% weight of a poly-isocyanate capped, linear or branched polysiloxane prepolymer and (b) 20-50% by weight of a linear polydialkyl or polydiphenyl-siloxane disilanol having terminal siloxanol groups.
U.S. Pat. No. 4,983,702 discloses a cross-linked siloxane-urethane polymer in the form of an ophthalmic device, which consists essentially of the reaction product of (a) a di- or poly-hydroxyalkyl substituted alkyl polysiloxane and (b) an aliphatic, cycloaliphatic or atomatic di- or tri-isocyanate, wherein the total number of hydroxyl groups in component (a) is stoichiometrically equivalent to the total number of isocyanate groups in component (b), and with the proviso that an effective cross-linking amount of (a) or (b) is present and possesses a functionality of greater than two.
U.S. Pat. No. 4,711,943 discloses a non-fibrous polymeric contact lens material having improved oxygen permeability and stability, said material comprising a monomer having a first portion for increasing wettability which is hydrophilic and includes a side chain functionality selected from —CO—N— or —O—CO—N— (such as an acrylamide), and a second portion for increasing oxygen permeability, said second portion including a siloxane.
One class of polymers which holds considerable promise for novel contact lens materials are PEG based polyurethanes, and the use of silicone-containing PEG based polyurethanes in the manufacture of contact lenses is known. In particular, WO2011/055108 discloses the use of silicone containing compounds in the preparation of polyurethane polymers useful in the manufacture of contact lenses. The use of silicone containing compounds to increase the oxygen permeability of the resultant polyurethane polymers is disclosed. However, it has been found that if too much silicone-containing compound is used in the manufacture of the polyurethane polymer, the polymer becomes opaque or semi-opaque, rendering the polymer unsuitable in the field of contact lenses.
The present invention seeks to provide new silicone-containing polyurethane-based materials that are suitable for use in the contact lens industry. The silicone-containing polyurethane-based materials of the invention are thermoplastic copolymers that exhibit exemplary physical properties, in particular in terms of oxygen permeability and light transmissibility. Advantageously, the thermoplastic materials described herein are suitable for use in conventional injection moulding apparatus, thereby enabling high throughput production of contact lenses.